This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The purpose of this study is to determine the safety and effectiveness of ABT-874 in patients with moderate to severe chronic plaque psoriasis. Two different doses will be studied and compared to placebo. A placebo is an inactive substance that is given just like the active drug, but does not contain ABT-874. This study will look at how well ABT-874 treats psoriasis compared to Placebo over a 52-week period.